[CodeGen] Fix a warning

This patch fixes:

  clang/lib/CodeGen/CGOpenMPRuntimeGPU.cpp:1662:8: error: unused
  variable 'ParallelReduction' [-Werror,-Wunused-variable]

Author: kazutakahirata

clang/lib/CodeGen/CGOpenMPRuntimeGPU.cpp

@@ -1668,6 +1668,7 @@ void CGOpenMPRuntimeGPU::emitReduction(
   if (Options.SimpleReduction) {
     assert(!TeamsReduction && !ParallelReduction &&
            "Invalid reduction selection in emitReduction.");
+    (void)ParallelReduction;
     CGOpenMPRuntime::emitReduction(CGF, Loc, Privates, LHSExprs, RHSExprs,
                                    ReductionOps, Options);
     return;